1. Field of the Invention
This invention relates to an apparatus for selecting and synchronous switching of television cameras used in close circuit television for a monitoring system.
2. Description of the Prior Art
In a monitoring television system used as one of information transmission systems, at least one television monitor is connected through an electronic switch to a plurality of television cameras and a plurality of transmission lines. Picture images picked up by one of a number of TV cameras are displayed on the TV monitor by switching operations carried out by the electronic switch. The electronic switch provides selection of any individual television camera or sequentially switches a plurality of the television cameras for displaying a picture image of one camera after another on the television monitor or for recording into a video recorder.
In such an information transmission system it is preferable to mutually lock the internal synchronizing signals of a plurality of television cameras and the switching time to an external synchronizing signal, in order to prevent the picture image on the monitor from being disturbed during and immediately after the switching operation from one television camera to another.
As devices for synchronizing a plurality of television cameras, there are known apparatuses for transmitting a vertical synchronizing signal, or a vertical synchronizing signal and a horizontal synchronizing signal, or a composite synchronizing signal used in a television system.
In any of the known devices of the type under discussion, as the transmitted synchronizing signal itself is a train of pulses, which can be easily influenced by noise, the transmission of a synchronizing signal requires the use of coaxial cables with their high shielding effect, which makes it costly for the systems with a plurality of television cameras.
Another known apparatus for synchronizing a plurality of television cameras, is an apparatus for transmitting an external synchronizing signal from an external synchronizing generator to the television cameras by injecting the external synchronizing signal into the video signal transmission line and locking an internal synchronizing signal generator of the television camera by means of the transmitted external synchronizing signal. Such an apparatus is disclosed in U.S. Pat. No. 4,603,352, the contents of which are incorporated herein by reference.
Furthermore, in such an information transmission system, the selector is disposed at the monitor site connected to a plurality of coaxial cables or other transmission lines such as twisted pair, or fiber optics for the transmission of the composite video signals. If a large number of cameras are used in such a system and the cameras are spread into distant locations, the need for a plurality of long stretched coaxial cables or other transmission lines makes such a system laborious and costly.